1. Field of the Invention
The present invention relates to an EL (electroluminescence) display device formed of a semiconductor element (an element using a semiconductor thin film) built into a substrate, and to an electronic device (an electronic device) having the EL display device as a display.
2. Description of the Related Art
Techniques of forming a TFT on a substrate have been widely progressing in recent years, and development of applications thereof to an active matrix type display device are advancing. In particular, a TFT using a polysilicon film has a higher electric field effect mobility (xcexcPE) than a TFT using a conventional amorous silicon film, and high speed operation is therefore possible. As a result, it becomes possible to perform pixel control, conventionally performed by a driving circuit external to the substrate, by the driving circuit formed on the same substrate as a pixel.
This type of active matrix display device has been in the spotlight because of the many advantages which can be obtained by incorporating various circuits and elements on the same substrate in this type of active matrix display device, such as reduced manufacturing cost, display device miniaturization, increased yield, and higher throughput.
In a monolithic type display device having a pixel portion and a driving circuit for driving the pixel portion on the same substrate, the driving circuit is formed in the periphery of the pixel portion, and therefore, compared to forming only the pixel portion on the substrate, the required substrate size becomes larger by the size of the driving circuit. Consequently, the number of display devices which can be cut out of one substrate changes with how small the exclusive surface area of the driving circuit can be made.
In particular, in a display device whose pixel portion has a diagonal of 1 inch or less, it is necessary to load the driving circuit on an extremely small substrate, and the exclusive surface area of the driving circuit imparts a large influence on the substrate size. However, regardless of the size of the pixel portion, the driving circuit functions are the same, and in order to form a circuit with identical functions in a very small area, various factors such as increasing the TFT characteristics and techniques of miniaturization become key points.
The present invention has been made in view of the problems above, and an object of the present invention is to further miniaturize an active matrix type EL display device, and to reduce manufacturing cost. In addition, another object of the present invention is to further miniaturize an electronic device provided with the active matrix type EL display device as a display, and to reduce manufacturing cost.
An EL element is formed for each pixel in an active matrix type EL display device. EL element refers to a light emitting element comprised of a cathode, an EL layer, and an anode here. The output light of the EL element (hereafter referred to as EL light) is output either from a substrate side or from the side opposite to the substrate. This is shown in FIGS. 6A and 6B.
In the structure of FIG. 6A, the EL element is formed of a pixel electrode (anode) made from ITO (indium tin oxide), an EL layer, and an MgAg electrode (cathode), in order from the bottom of the EL element. Further, the cathode itself is thin, and therefore a protecting electrode (an aluminum electrode here) is formed in order to protect, and at the same time to supplement, the cathode function. In this case the EL light is output from the side of the substrate on which a TFT is formed. Therefore, out of the entire pixel electrode surface area, the portion under which the TFT and wirings are not formed becomes an effective luminescing region.
On the other hand, in the structure of FIG. 6B, the EL element is formed of a pixel electrode (anode) made from an aluminum film, an MgAg electrode (cathode), an EL layer, and an ITO electrode (anode), in order from the bottom of the EL element. In this case, the EL light does not transmit through the pixel electrode, and therefore all of the light is output to the side opposite to the substrate (the top side of the EL display device). The entire surface area of the pixel electrode therefore becomes the effective luminescing region.
For the case of FIG. 6A it is thus important to form as few elements or wirings as possible under the pixel electrode. However, in the case of FIG. 6B, there is no relationship, no matter what is formed under the pixel electrode, which is an utterly dead space.
To clarify the main point of the present invention, the invention aims to effectively utilize the dead space under the pixel electrode in an active matrix type EL display device in which the EL element is made to emit light by a method like that of FIG. 6B. Specifically, a driving circuit for driving a pixel portion is formed under the pixel electrode of each pixel arranged in a matrix state in the pixel portion. In addition, other signal processing circuits (such as a wave divider circuit, a booster circuit, a xcex3 compensation circuit, memory, and a differential amplifier circuit), not only the driving circuit, may also be formed.
In other words, the circuits and elements conventionally formed in the periphery of the pixel portion are arranged in the dead space within the pixel portion, and the substrate surface area can be effectively utilized. Note that elements, such as a protecting element used as a countermeasure against ESD (electro-static degradation), are included as elements formed in the periphery of the pixel portion.
Furthermore, the present invention is not only applicable to the active matrix type EL display device, but is also applicable to an EL display device which has a driving circuit formed on the same substrate and has a pixel portion of a simple matrix type. In other words, the present invention is effective for EL display devices in which the EL light in the pixel portion is output to the side opposite that of the substrate, and in which other circuits or elements are formed on the substrate.